The band engineering of M-doped SnTe (M = Ga, In, and Tl) is investigated by using first-principles calculations. Being consistent with experimental measurements, our calculations found that Ga doping hardly changes the valence band, while In doping introduces an obvious resonant state near the Fermi level. The resonant state is demonstrated to be from the anti-bonding of In-s and Te-p orbitals. Unexpectedly, no resonant state was observed in Tl-doped SnTe, indicating the nonmonotonic behavior of the Ga-In-Tl series. We show that the absence of the resonant state in Tl-doped SnTe is due to the downward shift of the Tl-s orbital, which may be attributed to the effect of lanthanide contraction. The increase of the Seebeck coefficient in In-doped SnTe is numerically confirmed by Boltzmann transport calculations. Moreover, we find that the mutually matched resonant state location and valence band separation is the key precondition for the combination of the resonant state and band convergence in SnTe. A further enhanced Seebeck coefficient (∼230 μV K(-1)) and ZT value (1.8 at 920 K) are predicted in codoped SnTe by In-Hg, owing to the synergy of two kinds of band engineering.